1. Field of the Invention
The present invention relates to a signal sensing circuit, and more particularly, to a signal sensing circuit across integrated circuits.
2. Description of the Prior Art
Referring to FIG. 1, a schematic diagram illustrating a detecting circuit for a capacitive touch sensor is shown. When a driven electrode Tx1 provided with an AC driving signal, capacitive couplings will be created between the driven electrode Tx1 and sensed electrodes Rx1 and Rx2 overlapping with the driven electrode Tx1. Signals of the sensed electrodes Rx1 and Rx2 can be converted into output voltages V′ and V″ by a front-end current sensing circuit, respectively. The output voltages V′ and V″ are related to the impedance of a reference resistor R. Since background interferences affecting adjacent sensed electrodes will be similar, subtracting the signals of a pair of adjacent sensed electrodes can effectively cancel out the common-mode noise and increase the signal-to-noise (S/N) ratio. Thus, the output voltages V′ and V″ can be converted into respective digital signals thereof through an A/D converter and subtracted from each other, so as to create a digital signal representing the difference between the output voltages V′ and V″ by digital subtraction. The output voltages V′ and V″ can be converted into a digital signal representing the difference between the output voltages V′ and V″ using an A/D converter as a subtractor, in other words, by analog subtraction.
In order to effectively reduce the interferences, signals of all the sensed electrodes have to be received simultaneously in order to simultaneously eliminate the interferences of the common-mode noise. Thus, the signal of each sensed electrode is coupled to a pin of an integrated circuit (IC). Since the number of pins of the IC coupled with the sensed electrodes is fixed, in the case of a large touch sensor, the number of the sensed electrodes may exceed the number of pins of a single IC used for coupling with the sensed electrodes; as a result, several ICs have to be connected together in series.
Referring now to FIG. 2, when two ICs are connected in series, the signals of a pair of adjacent sensed electrodes may be generated by different ICs. Each IC will have its own manufacturing process variations. Even if the same element adopts the same design, the elements manufactured using the same design may still have variations among them. For example, the same resistors are designed with the same impedance, but the actual impedances after manufacturing may be different due to variations in the manufacturing process, which will influence the associated signals.
For example, an output voltage V′ generated based on an input current I′ is associated with a reference resistor R′, and an output voltage V″ generated based on an input current I″ is associated with a reference resistor R″, wherein the reference resistors R′ and R″ are supposed to be designed to have the same impedance. In the example of FIG. 1, the output voltages V′ and V″ are generated in the same IC, and the difference between the output voltages V′ and V″ is Vd. Assuming manufacturing process variations exist between the first integrated circuit IC1 and the second integrated circuit IC2, the impedance of the reference resistor R1 is k (a constant value) times that of the reference resistor R″, thus the difference between the output voltages V′ and V″ will not be Vd, creating an error. Such an error will vary with the degree of manufacturing process variations, which makes it difficult to determine or correct.
From the above it is clear that prior art still has shortcomings. In order to solve these problems, efforts have long been made in vain, while ordinary products and methods offering no appropriate structures and methods. Thus, there is a need in the industry for a novel technique that solves these problems.